1. Field of the Invention
The present invention relates to an electrostatic sensor apparatus including a parallel arrangement of a plurality of sensor circuits, each designed to detect a change in exterior capacitance and output the resultant detection signal.
2. Description of the Prior Art
The present applicant has proposed a high-precision electrostatic sensor apparatus in, e.g., Japanese Patent Application No. 1-126234, which is capable of detecting a change in a very small electrostatic capacitance of 1.times.10.sup.-5 pF.
As shown in FIG. 1, this apparatus includes an oscillation circuit 1, a high-impedance conversion circuit 1a connected to the output terminal of the oscillation circuit 1, a resonance circuit 2, a detection electrode 3 for detecting a change in electrostatic capacitance with respect the electrostatic capacitance of an object to be detected, a detection circuit 4, and an amplifier circuit 5. The oscillation circuit 1 and the resonance circuit 2 respectively include ceramic resonators as independent dielectric resonators. For example, as shown in FIG. 2, a resonant frequency f.sub.0 of the resonance circuit 2 is set to be slightly shifted from a fixed oscillation frequency f.sub.1 of the oscillation circuit 1, and the resonant frequency f.sub.0 is shifted by .DELTA.f in correspondence with a small capacitance change .DELTA.C detected by electrode 3. The electrostatic capacitance change .DELTA.C is then converted into an output voltage .DELTA.V. The voltage .DELTA.V is detected/amplified and extracted.
Recently, there has been a demand for parallel processing of detection signals based on a plurality of electrostatic capacitances detected by an electrostatic sensor apparatus. In order to realize such parallel processing of signals, a plurality of channels, each including a sensor circuit consisting of the above-mentioned components from the oscillation circuit 1 to the amplifier circuit 5, may be adjacently arranged.
When, however, a plurality of channels, each including a sensor circuit consisting of the components from the oscillation circuit 1 to the amplifier circuit 5, are to be adjacently arranged, even a slight difference between the oscillation frequencies of the respective channels can cause interference, e.g., resonance between the oscillation frequencies of the respective channels. Consequently, accurate signal processing cannot be performed.
In order to solve such a problem, the present applicant has proposed a multi-channel electrostatic sensor apparatus in Japanese Patent Laid-Open No. 1-261756, in which a plurality of sensor circuits 6a, 6b, and 6c are operated with a common oscillation circuit 1, as shown in FIG. 3. According to this apparatus, there is no possibility of a shift in oscillation frequency signals supplied from the single oscillation circuit 1 to the sensor circuits 6a, 6b, and 6c of the respective channels through a high-impedance conversion circuit 1a, thus solving the problem of interference caused by the above-mentioned oscillation frequency shift.
However, in the system for distributing/supplying oscillation frequency signals to resonance circuits 2 of the respective channels by using the common oscillation circuit 1, impedance mismatching is caused on the resonance circuit 2 for the oscillation circuit 1 side. The oscillation frequency supplied to the resonance circuit 2 of each channel then distortion due to the influence of a reflected wave from each impedance mismatching portion. And loss of resonant curve or error in an output from each resonance circuit 2. For example, the resonance circuit 2 of the sensor circuit 6a has oscillation frequency characteristics indicated by the resonant curve shown in FIG. 4, whereas the resonance circuit 2 of the sensor circuit 6b has oscillation frequency characteristics indicated by the resonant curve shown in FIG. 5. That is, outputs or the waveforms of signals output to the resonance circuits 2 of the respective channels differ from each other, and the sensitivities of the sensor circuits of the respective channels of the electrostatic sensor apparatus vary.